The present invention relates generally to the field of magnetic resonance imaging systems and, more particularly, to a technique for reducing acoustic noise in such systems.
Magnetic resonance imaging (MRI) systems have become ubiquitous in the field of medical diagnostics. Over the past decades, improved techniques for MRI examinations have been developed that now permit very high quality images to be produced in a relatively short time. As a result, diagnostic images with varying degrees of resolution are available to the radiologist that can be adapted to particular diagnostic applications.
In general, MRI examinations are based on the interactions among a static or primary magnetic field, a radio frequency (RF) magnetic field and time varying magnetic field gradients, with nuclear spins within the subject of interest. Specific nuclear components, such as hydrogen nuclei in water molecules, have characteristic behaviors in response to external magnetic fields. The precession of spins of such nuclear components can be influenced by manipulation of the fields to obtain RF signals that can be detected, processed, and used to reconstruct a useful image.
The magnetic fields used to produce images in MRI systems include a highly uniform, static main magnetic field that is produced by a primary magnet. A series of gradient fields are produced by a set of three gradient coils disposed around the subject. The gradient fields encode positions of individual volume elements or voxels in three dimensions. A radio frequency coil is employed to produce an RF magnetic field. This RF magnetic field perturbs the spin system from its equilibrium. Upon returning to the equilibrium after termination of the RF field, RF signals are emitted. Such emissions are detected by either the same transmitting RF coil, or by a separate receive-only coil. These signals are amplified, filtered, and digitized. The digitized signals are then processed using one of several possible reconstruction algorithms to reconstruct a useful image.
Many specific techniques have been developed for acquiring MR images for a variety of applications. One major difference among these techniques is in the way gradient pulses and RF pulses are used to manipulate the spin systems to yield different image contrasts, signal-to-noise ratios, and resolutions. Graphically, such techniques are illustrated as xe2x80x9cpulse sequencesxe2x80x9d in which pulses applied to the gradient and RF coils are represented along with temporal relationships among them. In recent years, pulse sequences have been developed which permit extremely rapid acquisition of large amounts of raw data. Such pulse sequences permit significant reduction in the time required to perform the examinations. Time reductions are particularly important for acquiring high resolution images, as well as for suppressing motion effects and reducing the discomfort of patients in the examination process.
While MRI systems excel in providing high quality images used in medical diagnostic applications, further improvement to the system is still needed. For example, by interaction of the pulses produced during an examination with the various mechanical and magnetic structures of the MRI scanner, acoustic noise can be produced which is transmitted both through the examination room in which the scanner is placed, as well as within the patient bore. Although the amplitudes and frequencies of the acoustic noise are not dangerous for the patient, the noise can be discomforting for patients and add to the anxiety of the examination. The noise can also be distracting and discomforting for clinicians and radiologists performing the examinations. While attempts have been made to reduce acoustic noise in MRI scanners, such as by changes in pulse sequences, further improvements are still needed. Specifically, although, as a general goal, noise at all frequencies emitted by MRI scanners should be further reduced, there is a particular need, at present, for techniques which can reduce noise in an audible range of approximately 15 Hz to approximately 15 kHz.
The present invention provides a novel technique for reducing acoustic noise emitted by an MRI scanner designed to respond to such needs. The technique makes use of energy absorbing laminate structures which can be placed in various locations in and around the scanner structure to reduce emitted acoustic noise. The technique is adaptable in many ways to provide for noise reduction both within the patient bore, within the gradient coil structures, and in and around the structures supporting the RF coil, the gradient coils, and the primary magnet. The technique may be adapted for new coils and scanners, and may also be adapted for retrofitting existing scanners, such as during regular or special servicing.
In an exemplary implementation, laminate tiles or subunits are made for fitting to the various structures in which noise dampening is desired. Laminate tiles may include a range of materials and composite structures, such as a structural support to which an energy absorbing material is applied. Multiple layers of material may be provided, where desired. The design and configuration of the individual subunits or tiles facilitates their application to the coil structures and to the housing and support structures of the scanner. The acoustic noise reducing tiles may thus be provided in a kit for retrofitting existing scanners.